Reduction potentials of pentaammine complexes of cobalt(III), rhodium(III) and iridium(III): physical correlations

The reduction potentials of a large range of [M(NH3)5L]π+ complexes (M = CoIII, RhIII, IrIII) have been determined in aqueous solution (μ 0.1 M, NaClO4). Irreversible reductions span the range from + 0.22 to -0.47 V for cobalt(III) complexes and from -0.86 to -1.45 V for rhodium(III),and occur near -1.7 V for iridium(III), determined against the standard (KCl) calomel electrode. Despite several reports to the contrary in the literature, no correlations exist of E� with absorption spectra (Co and Rh), nuclear magnetic resonance spectra [δ(1HN), Co, Rh, Ir, and δ (59Co)], vibrational spectra [v(Co-N)], or rates of aquation and base hydrolysis (Co, Rh, Ir). Complexes which undergo outer-sphere [e.g., with Ru(NH3)62+] and predominantly inner-sphere (e.g., with VZ+aq) homogeneous electron transfer can be distinguished by the presence or absence of a correlation of In kex with E� respectively, consistent with the expectations of the Marcus-Hush theory of outer-sphere electron transfer.